How to use cash back

The cash back you get from Apple Card can be used two ways. 1. applied to the balance of your Apple Card when making payments or 2. It gets added to your Apple Cash card and you can use it for purchases for your Apple ID or for purchases at stores that accept Apple Pay and Discover card.

Get unlimited Daily Cash with Apple Card - Apple Support

did you see this link already? - Set up and use Apple Cash

What you can do with Apple Cash

• Add money to Apple Cash. 
• Make purchases using Apple Pay. 
• Send money to someone in the Messages app.2
• Transfer money from Apple Cash to your bank account.
• Use your Apple Cash to make payments on your Apple Card balance.
• Receive Daily Cash from Apple Card purchases.3